Although it may be true that this will work, I don't think
that is the reason WHY they put it there.  Typically,
a manufacturer will put a reverse biased diode across
the power supply terminals as a form of transient
protection.  If a voltage spike appears on the positive
terminal greater than the reverse breakdown voltage
of the diode, the diode will bleed the excess to ground.
It may not be as good as a 79 cent transorb but for 7
cents the diode is ALOT better than nothing.

Mike DeMetz wrote:

> > Provide the simplest protection against accidental
> >     reverse connection (a series diode), mark the
> >     terminals clearly '+' and '-', color the terminals
> >     red and black respectively, and the intended User
> >     will understand what's required.
> >
> Another method I've seen on CBs is to connect a diode across the power
> terminals reversed biased. Connecting backwards blows the fuse.